The invention relates to a signal processing device comprising sampling means for sampling an input signal in the form of charge carrier packages and a shift register provided with an input region to which a signal sample is offered during operation, and with transport means for transporting the signal sample to an output region of the shift register.
Such a device is described in "PT Elektronica-Elektrotechniek" no. 4, April 1990, pp. 38-39, where the device is used in a memory oscilloscope for high-frequency input signals. In the known device, the shift register is formed by a charge-coupled device (CCD) comprising a series of gate electrodes situated one after the other and separated from a subjacent p-type semiconductor region by an insulating layer. The gate electrodes are connected to clocking means which during operation generate clock voltages by which local potential minima are generated in the semiconductor region. Such a potential minimum is formed by means of four gate electrodes situated next to one another and forms a cell in which a signal sample can be stored. The signal sample can then be shifted from cell to cell by the CCD under the influence of the clock voltages. The signal samples are shifted further by the CCD until the oscilloscope is triggered, which freezes the condition of the CCD. Subsequently, the discrete samples are read out from the shift register at a sufficiently low speed for reconstructing and displaying the input signal.
A signal which does not contain significant frequency components above f Hz can in principle be reconstructed from a series of samples thereof if the time interval between consecutive signal samples is smaller than 1/2.f second. This means that in proportion as the frequency of the input signal is higher, more signal samples are to be taken per unit time in order to enable a faithful reconstruction of the signal.
The known device in which each signal sample lies in storage below four gate electrodes of the CCD has the disadvantage that the size of the shift register will have to increase considerably in proportion as a greater frequency range of the is required. In addition, the upper limit of the frequency range is a fixed quantity for a device of the known type having a given number of gate electrodes.